Conventionally, a hydraulically-operated injection molding machine is known, which is so designed as to maintain the maximum internal mold pressure at a predetermined target value during a period for the filling of a mold with a molten resin, thereby preventing excessive or short filling. In the injection molding machine of this type, the internal mold pressure is subjected to closed-loop control by means of a hydraulic mechanism in accordance with a feedback signal indicative of the internal mold pressure and supplied from a pressure sensor disposed in a resin passage of the mold (Japanese Patent Publication No. 58-52486). However, the internal mold pressure sensor, which is arranged to be embedded in the resin passage, is high in cost. Moreover, since the internal mold pressure sensor is exposed to molten resin, it is liable to be lower in performance and have a shorter life. These drawbacks are particularly noticeable when the pressure sensor is embedded in a hot-runner mold. Furthermore, it is difficult to dispose the internal mold pressure sensor in the hot-runner mold which provides only a small embedding space for the sensor.
Also known is an electrically-operated injection molding machine which is so designed as to control the injection speed in an injection process to a target value which can be variably set in accordance with the axial moved position of an injection screw, control the hold pressure in a hold process to a target value in accordance with a detection output supplied from pressure detecting means, formed of an internal mold pressure sensor or a strain gauge, and optimize the timing for the switching from the injection process to the hold process (Japanese Provisional Patent Publication No. 62-218118). In the injection molding machine of this type, however, the injection pressure sometimes becomes improper. In this case, the quality of molded articles is degraded.